Easier Electroanalysis in Flowing Liquids
Carbon fibers could prove effective and versatile in the shifting world of liquid-flow electrochemical detection.
Jan Vacek and Jan Hrbac |
Progress in the field of flow electrochemical detectors has experienced a technological shift in the last decade – high-surface-area flow cells are being replaced with microelectrode systems, which are enabling applications of chip technologies and microfluidics.
Currently, flow electrochemical detectors (EDs) are used for the speciation, biochemical, environmental or industrial monitoring of a wide variety of analytes. The majority of these are electrochemical analyses with the use of large electrode systems. The main limitation of current flow systems is the low reproducibility of analyses, related to passivation of the working electrode surfaces and also to the lack of standardization for electrode regeneration procedures. Hence the reason why recent research and development in the field of flow EDs is oriented towards applications of microelectrode systems.
We believe that carbon fiber is a highly effective and not yet fully appreciated microelectrode material. Indeed, carbon fiber cylindrical microelectrodes (CFMEs) are mechanically highly resistant working electrodes with advantageous electrochemical properties. Regeneration of the CFME surface in between analyses can be primarily achieved by repeated optimized potential ramps. Potential-controlled regeneration of the CFME surface can be performed in two ways: i) mildly, so that only desorption of the undesirable oxidation products occurs, or ii) vigorously, which leads to mechanical restoration (ablation) of the surface (1).
CFMEs can be used in continuous flow systems, in which case the carbon fiber is installed directly into a PEEK capillary. CFMEs can be used without surface treatment but, if required, selectivity can be enhanced by applying appropriate layers of functional polymers that provide permeability for a target analyte that may be subsequently electrooxidized on the carbon fiber surface (2). Another way to ensure the selectivity of the determination is to modify the surface of the carbon fiber with metal layers, metal oxide layers, or semiconducting components, preferentially in the form of nanostructured deposits (3). Metal-modified CFMEs enable analysis not only in the positive but also in the negative potential region, which enables the analysis of substances undergoing reductive transformations.
From an application point of view, a microelectrode-based flow ED can be used in conjunction with flow injection analysis, capillary electrophoresis, liquid chromatography and microdialysis techniques. But we see great application potential in the interconnection of CFMEs with microfluidics, chips and microseparation techniques; for example, a microcolumn adapted for solid-phase extraction. Microelectrode systems need potentiostats working in the current range of <1 nA – a requirement that is met today by a wide range of devices, such as the experimental LabFlow sensing platform fully adopted for CFME applications. Certainly, analysis in flowing fluids – and the subsequent data evaluation – has specific software requirements, but these have already been met by a number of software applications. Indeed, we recently developed the freeware – eL-Chem Viewer – in our lab for this very purpose (4).
Electrochemical analysis in flowing liquids is moving beyond the concept of large-area electrodes, with research and development interest turning to microelectrodes that can be suitably modified and adapted to perform specific analytical tasks. In the future, one can also expect applications of microelectrode systems to grow as a field, appropriately modified with semiconducting polymers, microelectronic and optoelectronic components (5, 6). Given the potential applications and the versatility of advanced microelectrodes, we can see a bright future for the field of electrochemical detection.
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